* DESCRIPTION (provided by applicant): Diet is known to impact DNA methylation, however, little is know about which dietary components are linked to cancer prevention. Human milk is the optimal and evolutionarily designed source of human infant nutrition;the American Academy of Pediatrics recommends that infants receive only human milk for the first 6 months of life. Breastfeeding is associated with reduced risk of childhood cancers including acute lymphoblastic leukemia. Levels of key dietary components known to affect methylation differ in human milk and infant formula, and accordingly, in infants fed human milk or formula. Goals: 1) To assess the effect of early feeding on aberrant methylation of 40 tumor suppressor genes in 100 infants (50 exclusively breastfed infants, and 50 exclusively formula fed infants) at 4 months. 2) To perform an exploratory genome-wide analysis of DNA from 10 infants who were exclusively breastfed for 4 months, compared with 10 exclusively formula fed infants, using methylation microarrays interrogating 18,000 gene promoters. Research Design and Methods: We will enroll 100 newborns and obtain blood samples at birth and 4 months, using infants already being enrolled in a funded cohort study. DNA from peripheral blood lymphocytes of infants who breastfeed exclusively for 4 months will be compared with those who formula feed only for 4 months. The primary analysis will be via methylation-specific real-time PCR for promoter methylation in 40 tumor suppressor genes associated with cancer, including childhood leukemia. Statistically significant differences in promoter methylation profiles will be assessed. The percent of tumor suppressor genes substantially methylated will be quantified in the two populations. Significance: Given that methylation is a potentially reversible DNA change, detecting a potential molecular link between early methylation changes, infant feeding, and childhood cancer development would have major implications for cancer prevention strategies. 1 PUBLIC HEALTH RELEVANCE: An association exists between breastfeeding and reduced risk of childhood leukemia, but no causal pathway has been determined. Levels of key dietary components known to affect methylation differ in human milk and infant formula. The present application will assess the effect of exclusive breastfeeding versus exclusive formula feeding on aberrant methylation of 40 tumor suppressor genes in 100, 4-month old infants, and will perform an exploratory genome-wide analysis of DNA from 20 infants using methylation microarrays interrogating 18,000 gene promoters. 1